The present invention relates to article of cookware, and in particular to cookware vessels fabricated at least in part from copper
Copper cookware is appreciated for its high thermally conductivity as well as the reddish-gold appearance. This reddish-gold appearance is unique from other metals with the exception of gold and copper alloys, such as brass and bronze, and thus readily distinguishes such cookware. Although copper cookware is both beautiful and considered to offer superior cooking performance, due to the high thermal conductivity, it tarnishes readily on exposure to heat as well as liquids (including the moisture residue on the fingers to create finger prints). Accordingly, because of the extra effort required to clean such tarnish, copper cookware is not popular with some consumers despite its other benefits.
Prior methods of minimizing the tarnishing of copper cookware include burying the copper layers in at least one of interior and exterior metal cladding so it is not visible, as for example in U.S. Pat. No. 6,267,830 (issued Jul. 31, 2001 to Groll). While this provides most of the thermal performance benefits of copper it eliminates the secondary benefits of its unique and elegant appearance.
Another partial attempt to solve this problem is provided by U.S. Pat. D474,936 (issued on May 27, 2003 to Groll) wherein a indented golden copper colored band formed around the outer periphery on a lower portion of a cooking vessel as decorative feature. Although this design exposes some copper metal, it still does not prevent tarnishing. Further, such a groove tends to collect dirt and debris and is more difficult to clean out than a largely straight sided cooking vessel.
Although protective coatings have been applied to copper to prevent tarnishing, they suffer from two deficiencies. A variety of inorganic coatings have been suggested to apply in place of or over copper, such as transition metal nitrides, oxy-nitrides and carbides as they can achieve different shades of reflective golden metal appearances. However, such inorganic coatings are both expensive and not necessary thermally and chemically stable in cooking at high temperature, where they can be exposed to direct flames. It has been found that such coatings, when applied as a thin layer over copper merely temporally minimize the development of a tarnished appearance, which eventually occurs as the underlying copper oxidizes at high temperatures. Eventually this oxidized copper becomes visible, starting first at the bottom of the cookware, which undergoes multiple cycles of exposure to high temperature.
Organic lacquers, while capable of preventing tarnishing on decorative copper items do not withstand the high temperatures that are reached by functional cookware. This is particularly a problem at the bottom of the cookware article that is exposed either directly to the flame, but also of issue somewhat more distal from the heater element, as the copper is a good conductor of heat.
It is therefore a first object of the present invention to provide clad cookware that exposes copper layers that are easy to clean and do not trap dirt.
It is a further object of the invention to provide exposed copper layers that do not tarnish under normal use conditions, maintaining the beauty of copper cookware, yet without the high maintenance of cleaning tarnished copper.
Yet another object of the present invention is to provide a simpler and lower cost fabrication method for creating clad cookware that exposes such copper layers.